Angkor (Khmer: អង្គរ or នគរ, "Capital City") is a region of Cambodia that served as the seat of the Khmer Empire, which flourished from approximately the 9th to 15th centuries. The word Angkor is derived from the Sanskrit nagara (नगर), meaning "city". The Angkorian period began in AD 802, when the Khmer Hindu monarch Jayavarman II declared himself a "universal monarch" and "god-king", and lasted until the late 14th century, first falling under Ayutthayan suzerainty in 1351. A Khmer rebellion resulted in the 1431 sacking of Angkor by Ayutthaya, causing its population to migrate south to Longvek.The ruins of Angkor are located amid forests and farmland to the north of the Great Lake (Tonlé Sap) and south of the Kulen Hills, near modern-day Siem Reap city (13°24′N, 103°51′E), in Siem Reap Province. The temples of the Angkor area number over one thousand, ranging in scale from nondescript piles of brick rubble scattered through rice fields to the magnificent Angkor Wat, said to be the world's largest single religious monument. Many of the temples at Angkor have been restored, and together, they comprise the most significant site of Khmer architecture. Visitors approach two million annually, and the entire expanse, including Angkor Wat and Angkor Thom is collectively protected as a UNESCO World Heritage Site. The popularity of the site among tourists presents multiple challenges to the preservation of the ruins.In 2007, an international team of researchers using satellite photographs and other modern techniques concluded that Angkor had been the largest pre-industrial city in the world, with an elaborate infrastructure system connecting an urban sprawl of at least 1,000 square kilometres (390 sq mi) to the well-known temples at its core. Angkor is considered to be a "hydraulic city" because it had a complicated water management network, which was used for systematically stabilizing, storing, and dispersing water throughout the area. This network is believed to have been used for irrigation in order to offset the unpredictable monsoon season and to also support the increasing population. The closest rival to Angkor, the Mayan city of Tikal in Guatemala, was between 100 and 150 square kilometres (39 and 58 sq mi) in total size. Although the size of its population remains a topic of research and debate, newly identified agricultural systems in the Angkor area may have supported up to one million people.